The present invention relates to a line pressure control for a V-belt type continuously variable transmission.
There is known a V-belt type continuously variable transmission including a fluid coupling with a lock-up clutch for improving start-up performance of the vehicle which the transmission is mounted to. A transmission of the above kind is disclosed in Japanese Utility Model First Publication No. 62-153466.
The start-up performance is, however, not satisfactory since no torque multiplication is made by the fluid coupling. If the fluid coupling is replaced with a torque converter, it is necessary to increase the line pressure in accordance with an increase of torque due to the torque multiplication by the torque converter. Otherwise, there occurs a slip between a V-belt and the associated pulleys at least one of which is subject to the line pressure to grip the V-belt. If in order to solve this problem the line pressure is increased to a level high enough to cause the pulley to grip the V-belt firmly when the torque converter effects the torque multiplication, an excessively large gripping force is applied to the V-belt when the torque converter ceases its torque multiplication function. This causes a degradation of endurability of the V-belt. If the line pressure is maintained high, the energy loss of an oil pump increases accordingly.
An object of the present invention is to solve the above-mentioned problems.
A specific object of the present invention is to provide a line pressure control system for a V-belt type continuously variable transmission wherein the line pressure is increased whenever a torque multiplication is expected.